The present invention relates to a compressed gas-blast circuit breaker having two coaxially arranged contact pieces which engage one another in a switched-on position, as well as a blast nozzle and pump cylinder for pumping arc extinguishing gas.
A compressed gas-blast circuit breaker of this type is disclosed in German patent document 3,942,489. Its pump cylinder, which is moved along with a movable first contact piece, surrounds a constant-volume blast chamber and a pumping space which is likewise surrounded by the pump cylinder and is connected to the blast chamber via a non-return valve. The pumping volume of the pumping space can be reduced upon switching off by means of a piston arranged in the pump cylinder in order to pump arc extinguishing gas into the blast chamber through the non-return valve for the purpose of building up blast pressure. During a switch-off stroke, the pump piston is retained by a toggle lever blocked in a straightened position in a position of such a dimension that the pumping volume vanishes when the minimum distance required between the contact pieces for the arc extinguishment is reached. Directly after this contact position has been reached, the lock is released, with the result that the pump piston can move together with the pump cylinder in the switching-off direction until the movable first contact piece has reached the switched-off position. Upon switching on, the underpressure built up in this case in the pumping space draws the pump piston in the direction of the switching-on movement until the toggle lever is again located in the straightened position and is locked there again. A ventilating valve in the pump piston must be spring-loaded for this purpose in such a way that it does not open until after the locked position has been reached, in order to fill the pumping volume again witch arc extinguishing gas. In this known compressed gas-blast circuit breaker, the return of the pump piston into the locked position is not ensured upon switching on. If, for any reason, for example increased friction, the forces necessary for this purpose increase, the ventilating valve can open prematurely. In this switched-on position of the compressed gas-blast circuit breaker the pump piston can then be located in an arbitrary, non-locked position, and the consequence of this is that when switching off is next performed no blast pressure has built up during separation of the contact pieces. Since the ventilating valve has to be spring-loaded, there is always a corresponding difference present between the pressure in the pumping volume and the pressure in the surrounding space even when switching on. This requires additional drive energy.
Furthermore, German patent document 3,942,489 discloses a compressed gas-blast circuit breaker with a pump cylinder surrounding only a pumping space. In order to have the driving force available at the start of a switch-off stroke, as it were, exclusively for accelerating the moving circuit breaker parts, and not to use it with the compression of the arc extinguishing gas until towards the end of a stroke segment, that is to say when the moving circuit breaker parts are already in motion, the pump piston is moved in the same sense as the pump cylinder at the start of the switch-off stroke. Subsequently, for the purpose of quickly compressing the arc extinguishing gas in order to enable powerful blasting of the arc, the direction of movement of the reciprocating piston is reversed so that it moves in the opposite sense to the pump cylinder. After a repeated reversal of a direction of movement, it finally moves again in the same sense as the pump cylinder. In order to control the pump piston, the latter is supported by a toggle lever which is controlled by a rocker which is pivoted at one end to the toggle joint and at the other end to a point which is moved along with the movable contact piece. At the start of the switch-off stroke, the rocker presses the toggle joint outwards, as a result of which the bent position, already present in the switched-on position, of the toggle lever is further intensified. In this case, the pump piston moves in the same sense as the pump cylinder. Subsequently, the rocker draws the toggle joint inwards, as a result of which the bent position is reduced. As a result, the pump piston moves in the opposite sense to the pump cylinder until the toggle lever is completely straightened, in order to quickly compress the arc extinguishing gas. After the straightened position has been passed, the bend is intensified again, as a result of which the pump piston is moved in the same sense as the pump cylinder. The pump piston is located approximately in the same position both in the switched-on and in the switched-off position of the movable contact piece.
A further compressed gas-blast circuit breaker includes a pump cylinder which is moved along with the movable contact piece and has both a blast chamber and a pumping space connected to the latter via a non-return valve, as disclosed in German patent document 3,942,489. In order to pump arc extinguishing gas from the pumping space into the blast chamber in order to build up the blast pressure at the start of a switch-off stroke, the blast piston, which together with the blast cylinder delimits the pumping space, is restrained by a parallelogram-type linkage and a contour, acting as a slotted link, of the movable contact piece, and is unlocked after this contact piece has traversed a stroke segment, with the result that it can move in the same sense as the pump cylinder. A spring acts between the pump cylinder and the pump piston in order to increase the pump volume again upon switching on. Loading the spring at the same time as compressing the arc extinguishing gas requires increased driving force and considerable drive energy. Moreover, upon switching on, the blast piston can be carried along with the blast cylinder as a consequence of friction, with the result that compression of the arc extinguishing gas is no longer possible at the next switching off.